Repair of composite sandwich structures with uneven bond surfaces

ABSTRACT

A method of restoring a section of a component having a honeycomb bonded to a skin includes the steps of separating a first honeycomb from the skin, wherein the honeycomb includes an uneven surface. At least a portion of the uneven surface is removed to form a bonding surface on the skin. A conformable layer is bonded to the uneven surface. Then a replacement honeycomb is then bonded to the conformable layer.

RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.11/441,866, which was filed May 26, 2006, now U.S. Pat. No. 7,572,347.

BACKGROUND OF THE INVENTION

This invention relates to composite materials and, more particularly, torepairing composite sandwich structures.

Composite structures are commonly known and used in, for example, gasturbine engines for aerodynamic fairings. One type of composite is asandwich structure having a face skin and a back skin that are bonded toa honeycomb core. The face skin and back skin may be laminatedstructures with a resin matrix, such as epoxy, with one or more plies ofreinforcement fibers.

Laminated skins are fabricated in a process that involves compacting theresin filled plies of reinforcing fiber together while curing the resin.The texture of the tooling materials used on each molded face of thelaminate during compaction and cure will be observed on the resultinglaminate surface. If one or more of the molded faces is restrained by anon-rigid material, textures may telegraph thru the thickness of thelaminate. During compaction against the ends of the honeycomb locallypress into the skins and form a dimpled imprint surface.

In some instances, it is desirable to locally replace a honeycomb thathas become damaged from an impact, elevated temperatures, abrasion,erosion, or other phenomenon rather than replace the entire compositestructure. The honeycomb is machined to have a flat surface on bothsides. The flat surfaces do not conform well with uneven surfaceswithout crushing the honeycomb. It is unfeasible to have the ends of thereplacement honeycomb align perfectly with the dimpled imprints left bythe removed honeycomb section. Traditionally, the damaged honeycomb isremoved and an adhesive is applied to the laminate dimpled imprintsurface to bond the skin and a replacement honeycomb together.Typically, an additional thickness of adhesive is used to “absorb” theunevenness of the dimpled imprints. However, adhesive bond strengthrapidly declines as thickness increases, and structural requirementswill limit the simple application of extra adhesive. Thus, there is aneed for a repair method that provides improved bonding between therepair honeycomb and uneven laminates. This invention addresses thoseneeds while avoiding the shortcomings and drawbacks of the prior art.

SUMMARY OF THE INVENTION

An example method of restoring a section of a component having ahoneycomb bonded to a skin includes the steps of separating the facesheet and the original honeycomb from a skin, wherein the skin includesan uneven surface. At least a portion of the uneven bond surface isthinned to form a more even bonding surface on the laminate. Aconformable is applied over the bond surface. A replacement honeycomb isthen pressed into the conformable layer. The conformable layer and theadhesives are cured to restore original component structure.

An example composite component includes a sandwich structure with ahoneycomb, a laminate that is bonded to each side of the honeycomb, anda repair section. The repair section includes an uneven laminate fromthe original component, a repair piece of honeycomb, a replacementlaminate, a conformable layer, and adhesives to bond the repair sectionto itself and the adjacent areas.

BRIEF DESCRIPTION OF THE DRAWINGS

The various features and advantages of this invention will becomeapparent to those skilled in the art from the following detaileddescription of the currently preferred embodiment. The drawings thataccompany the detailed description can be briefly described as follows.

FIG. 1 illustrates an example composite structure having a damagedhoneycomb core.

FIG. 2 illustrates an example method for repairing the compositestructure of FIG. 1.

FIG. 3 illustrates a view of the composite structure after removing thedamaged honeycomb core and making the bond surface less uneven.

FIG. 4 illustrates a view of the composite structure with a repairmaterial section.

FIG. 5 illustrates the composite structure after the repair process.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 illustrates selected portions of an example composite structure20, such as an acoustic composite used in a fan-bypass area or nacelleof a gas turbine engine for noise attenuation. In this example, thecomposite structure 20 includes a face skin 22, which may includeperforations 23 that extend through the thickness of the face skin. Theface skin 22 and a back skin 24 are bonded to a honeycomb core 26 toprovide a composite sandwich structure. In the disclosed example, eachof the face skin 22 and the back skin 24 include composite layers 28,such as layers made of a resin matrix with continuous woven fiberreinforcement that are concurrently bonded to and compacted against theends of honeycomb core 26.

In the illustrated example, the honeycomb core 26 has become damaged by,for example, an impact, elevated temperatures, abrasion, erosion, orother phenomenon. As will be described below, the composite structure 20is repaired by removing the damaged honeycomb core 26 and replacing itwith a replacement honeycomb core 26′ (FIG. 4). The replacementhoneycomb core 26′ is bonded to the back skin 24 to provide astructurally durable repair that is dimensionally similar to theoriginal dimensions of the composite structure 20.

Referring to the example shown in FIG. 2, the composite structure 20 isrepaired in several steps. In this example, the damaged face skin 22 andhoneycomb core 26 are locally removed from the back skin 24 at step 42.Although the disclosed example describes removal from and bonding to theback skin 24, it is to be understood that the principles of the repairare also applicable to the face skin 22. Removal of the damagedhoneycomb core 26 reveals an uneven surface 45 on the back skin 24caused by the original manufacture of the back skin 24. At step 44, theuneven surface 45 is partially leveled to produce a more even bondingsurface 47. At step 46, the replacement honeycomb core 26′ is bonded tothe bonding surface 47 of the back skin 24. As will be described below,the disclosed method provides the benefit of reducing surface variationon the back skin 24 to enable formation of a stronger bond between thereplacement honeycomb core 26′ and the back skin 24.

FIG. 3 shows the back skin 24 after the removal step 42. As can beappreciated, the back skin 24 includes the uneven surface 45. In thisexample, the uneven surface 45 is a product of the fabrication processof the original composite structure 20. In this example, the compositestructure 20 was compacted and cured in a known manner, which resultedin the uneven surface 45. During the fabrication process, ends 50 of thehoneycomb core 26 press into the back skin 24 and form the dimpledimprint illustrated in FIG. 3. In this example, the dimpled imprintincludes compacted (compressed/consolidated) sections 52 where the ends50 of the honeycomb core 26 pressed into the back skin 24, andrelatively uncompressed sections 54 that were between the ends 50 andtherefore not compacted as much.

After the honeycomb core 26 removal step 42, a portion 56 of the unevensurface 45 is removed to produce the bonding surface 47. The portion 56can be removed in a variety of different ways. In one example, theportion 56 is mechanically removed using a grinding, sanding, or otherknown mechanical process.

In the illustrated example, the back skin 24 includes three fiberglasslayers 28 with a cured epoxy resin matrix. In this example, the highspots of the top composite layer 28 (i.e., the top layer in FIG. 3) arelocally removed such that the middle composite layer 28 is at leastpartially exposed on the bonding surface 47. The removal of the portion56 from the back skin 24 provides the benefit of reducing surfacevariation on the uneven surface 45. This provides a flatter surface tobond to, and thereby allows a stronger bond to be formed between thebonding surface 47 and the replacement honeycomb core 26′. In otherexamples, more than one composite layer 28 or less than one compositelayer 28 may be removed, depending on the desired reduction in surfacevariation.

After removing a portion of the uneven surface 56, a conformable repairmaterial 66 is placed onto the bonding surface 47. As illustrated inFIG. 4, a layer of adhesive 68 a may optionally be used between therepair material 66 and the bonding surface 47 to strengthen the bondbetween the back skin 24 and the repair material 66. In this example,the repair material 66 is a composite layer 28′ that is similar to oridentical to the composite layers 28 used in the back skin 24, exceptthat it is not compacted and the resin is not cured. For example, theconformable repair material 66 is made of an uncured epoxy resin matrixwith continuous woven fiberglass reinforcement. Using the compositelayer 28′ provides the benefits of absorbing tolerance variation betweenthe original honeycomb and repair material 66, providing additionalsmoothing for intimate contact with the honeycomb ends 50, restoringstructural strength lost by removal of the uncompressed portions 56, andthicker regions of adhesive thickness are interrupted by the fiberreducing the strength debit due to adhesive thickness.

Optionally, another layer of adhesive 68 b is used on top of theconformable repair material 66 to promote bonding between theconformable repair material 66 and the replacement honeycomb core 26′.Once the conformable repair material 66, and optionally the adhesivelayers 68 a and 68 b, are in place, the replacement honeycomb core 26′and is pressed, against the conformable repair material 66. The faceskin 22 is then installed over the replacement honeycomb 26 in a knownmanner

In the disclosed example, compaction of the face skin 22, thereplacement honeycomb core 26′, and the back skin 24 occurs underpressure at an elevated temperature to cure the resin of the conformablerepair material 66 and adhesive layers 68 a and 68 b. During compactionand curing, the ends 50′ of the replacement honeycomb core 26′ compressportions of the repair material 66 immediately below the ends 50′, whilethe repair material 66 between the ends 50′ is relatively uncompressed,mimicking the original sandwich structure.

FIG. 5 illustrates the composite structure 20 after the repair process.In this example, the repair material 66 is fully cured and compacted toa dimension d₁ that is about equal to a dimension d₂ (FIG. 1) and adimension c₁ that is about equal to a dimension c₂ (FIG. 1) of thecomposite sandwich structure 20 before the repair (e.g., before damageto the honeycomb core 26). Thus, the disclosed example method of repairprovides a structurally durable repair section that is dimensionally andfunctionally equal with surrounding areas of the composite sandwichstructure 20 that have not been repaired.

Although a preferred embodiment of this invention has been disclosed, aworker of ordinary skill in this art would recognize that certainmodifications would come within the scope of this invention. For thatreason, the following claims should be studied to determine the truescope and content of this invention.

1. A composite component comprising: a honeycomb; a skin bonded to thehoneycomb; and a repair section made of a composite material having amatrix material and reinforcing fibers, wherein the repair section bondsthe honeycomb and the skin together, wherein the repair section includesa compressed section and an uncompressed section.
 2. The component asrecited in claim 1, wherein the reinforcing fibers are continuousreinforcement fibers, and the matrix material is a resin matrix.
 3. Thecomponent as recited in claim 2, wherein the repair section includes afirst adhesive layer between the skin and the repair material.
 4. Thecomponent as recited in claim 3, wherein the repair section includes asecond adhesive layer between the honeycomb and the repair material. 5.The component as recited in claim 1, wherein the compressed sectioncorresponds to an end of the honeycomb adjacent the skin and theuncompressed section is spaced from the end of the honeycomb.
 6. Thecomponent as recited in claim 1, wherein the compressed section and theuncompressed section define an uneven surface.
 7. The component asrecited in claim 6, wherein a portion of the uneven surface ismechanically removed.
 8. The component as recited in claim 1, whereinthe skin has a top fiberglass layer, a middle fiberglass layer, and abottom fiberglass layer, each layer with a cured epoxy resin matrix. 9.The component as recited in claim 8, wherein the top layer is adjacentthe honeycomb, wherein the top layer is removed such that the middlelayer is at least partially exposed to a bonding surface.
 10. Thecomponent as recited in claim 1, wherein the component forms a portionof a gas turbine engine.
 11. The component as recited in claim 1,wherein the compressed section and uncompressed section define a peakand a valley.
 12. The component as recited in claim 1, wherein thecompressed section is at a different dimension than the uncompressedsection relative to the honeycomb.